1. Field of the Invention
This invention relates to an insulating glazing unit and a method of making same and, in particular, to an insulating glazing unit having an edge assembly to provide the unit with a low thermal conducting edge, i.e. high resistance to heat flow at the edge of the unit.
2. Discussion of Available Insulating Units
It is well recognized that insulating glazing units reduce heat transfer between the outside and inside of a home or other structures. A measure of insulating value generally used is the "U-value". The U-value is the measure of heat in British Thermal Unit (BTU) passing through the unit per hour (Hr)-square foot (Sq. Ft.)-degree Fahrenheit (.degree.F) ##EQU1##
As can be appreciated the lower the U-value the better the thermal insulating value of the unit, i.e. higher resistance to heat flow resulting in less heat conducted through the unit. Another measure of insulating value is the "R-value" which is the inverse of the U-value. Still another measure is the resistance (RES) to heat flow which is stated in Hr-.degree.F per BTU per inch of perimeter of the unit ##EQU2##
In the past the insulating property, e.g. U-value given for an insulating unit was the U-value measured at the center of the unit. Recently it has been recognized that the U-value of the edge of the unit must be considered separately to determine the overall thermal performance of the unit. For example, units that have a low center U-value and high edge U-value during the winter season exhibit no moisture condensation at the center of the unit, but may have condensation or even a thin line of ice at the edge of the unit near the frame. The condensation or ice at the edge of the unit indicates that there is heat loss through the unit and/or frame i.e. the edge has a high U-value. As can be appreciated, when the condensate or water from the melting ice runs down the unit onto wooden frames, the wood, if not properly cared for, will rot. Also, the larger temperature differences between the warm center and the cold edge can cause greater edge stress and glass breakage. The U-values of framed and unframed units and methods of determining same are discussed in more detail in the section entitled "Description of the Invention."
Through the years, the design of and construction materials used to fabricate insulating glazing units, and the frames have improved to provide framed units having low U-values. Several types of units presently available, and center and edge U-values of selected ones, are considered in the following discussion.
Insulating glass edge units which are characterized by (1) the edges of the glass sheets welded together, (2) a low emissivity coating on one sheet and (3) argon in the space between the sheets are taught, among other places, in U.S. patent application Ser. No. 07/468,039 assigned to PPG Industries, Inc. filed on Jan. 22, 1990, in the names of P. J. Kovacik et al. and entitled "Method of and Apparatus for Joining Edges of Glass Sheets, One of Which Has an Electroconductive Coating and the Article Made Thereby." The units taught therein have a measured center U-value of about 0.25 and a measured edge U-value of about 0.55. Although insulating units of this type are acceptable, there are limitations. For example, special equipment is required to heat and fuse the edges of the glass sheets together, and tempered glass is not used in the construction of the units.
In U.S. Pat. No. 4,807,439 there is taught an insulting unit marketed by PPG Industries, Inc. under the registered trademark SUNSEAL. The unit has a pair of glass sheets spaced about 0.45 inch (1.14 centimeters) apart about an organic edge assembly and air in the compartment between the sheets. A unit so constructed is expected to have a measured center U-value of about 0.35 and an edge U-value of about 0.59. Although providing insulating gas e.g. argon in the unit would lower the center and edge U-values, the argon in time would diffuse through the organic edge assembly raising the center and edge U-values to those values previously stated.
The unit of U.S. Pat. No. 4,831,799 has an organic edge assembly and a gas barrier coating, sheet or film at the peripheral edge of the unit to retain argon in the unit. The thermal performance of the unit is discussed in column 5 of the patent. U.S. Pat. Nos. 4,431,691 and 4,873,803 each teach a unit having a pair of glass sheets separated by an edge assembly having an organic bead having a thin rigid member embedded therein. Although the units of these patents have acceptable U-values, they have drawbacks. More particularly, the units have a short length, high resistance diffusion path. The diffusion path is the distance that gas, e.g. argon, air, or moisture has to travel to exit or enter the compartment between the sheets. The resistance of the diffusion path is determined by the permeability, thickness and length of the material. The units taught in U.S. Pat. Nos. 4,831,799; 4,431,691 and 4,873,803 have a high resistance, short diffusion path between the metal strip or spacing means and the glass sheets; the remainder of the edge assembly has a low resistance, long length diffusion path.
In U.S. Pat. No. 3,919,023, there is taught an edge assembly for an insulating unit that provides a high resistance, long length diffusion path that may be used to minimize the loss of argon. A limitation of the edge assembly of the patent is the use of a metal strip around the outer marginal edges of the unit. This metal strip conducts heat around the edge of the unit, and the unit is expected to have a high edge U-value.
It was mentioned that the effect of the frame U-value on the window edge U-value should be taken into account; however, a detailed discussion of frames having low U-value is omitted because the instant invention is directed to an insulating glazing unit that has low center and edge U-values, is easy to construct, does not have the limitations or drawbacks of the presently available insulating glazing units, and may be used with any frame construction.